For several decades, heat-shrinkable packaging articles have been used for the packaging of a variety of products. Food, particularly meat, has been vacuum packaged in such packaging articles. Through the years, these heat-shrinkable packaging articles have developed higher impact strength and higher seal strength, while simultaneously becoming easier to seal, having improved oxygen and moisture barrier properties, and having higher total free shrink at lower temperatures. High seal strength, high impact strength, and high puncture-resistance are particularly important for the packaging of fresh meat products, as leaking packages are less desirable to consumers and retailers alike. Moreover, leaking packages reduce shelf life by allowing atmospheric oxygen and microbes to enter the package. As a result, the packaging articles used for food packaging, particularly meat packaging, have evolved into being quite tough, and therefore difficult to open. Accordingly tailored films and/or tear propagation initiators have been developed to make these packages easy to open.
US2009116768 discloses multilayer barrier heat shrinkable films which, after being formed as bags or pouches, can be manually opened without the use of a knife or scissors or any other implement. According to this patent application these films are easy tearable due to the presence of at least one layer containing at least one incompatible polymer blend selected from the group consisting of:
(A) a blend of from 90 to 30 weight percent ethylene homopolymer and/or ethylene/alpha-olefin copolymer with from 10 to 70 weight percent ethylene/unsaturated ester copolymer having an unsaturated ester content of at least 10 weight percent;
(B) a blend of ionomer resin with ethylene/unsaturated ester copolymer, and/or polybutylene, and/or propylene homopolymer and/or propylene copolymer
(C) a blend of homogeneous ethylene/alpha-olefin copolymer with recycled polymer blend comprising ethylene homopolymer, propylene homopolymer, ethylene copolymer, propylene copolymer, polyamide, ethylene/vinyl alcohol copolymer, ionomer resin, anhydride-modified ethylene/alpha-olefin copolymer, and antiblocking agent; (D) a blend of ethylene/unsaturated ester copolymer with polypropylene and/or propylene/ethylene copolymer, and/or polybutylene, and/or modified ethylene/alpha-olefin copolymer, and/or styrene homopolymer, and/or styrene/butadiene copolymer;
(E) a blend of ethylene/norbornene copolymer with ethylene/unsaturated ester copolymer and/or polypropylene and/or polybutylene; (F) a blend of ethylene/alpha-olefin copolymer with polypropylene and/or polybutylene and/or ethylene/norbornene;
(G) a blend of homogeneous propylene homopolymer and/or homogeneous propylene copolymer with homogeneous ethylene/alpha-olefin copolymer and/or ethylene/unsaturated ester copolymer;
(H) a blend of propylene homopolymer and/or propylene/ethylene copolymer and/or polybutylene with ethylene/methyl acrylate copolymer and/or ethylene/acrylic acid copolymer and/or ethylene/butyl acrylate copolymer;
(I) a blend of polyamide with polystyrene and/or ethylene/alpha-olefin copolymer and/or ethylene/vinyl acetate copolymer and/or styrene/butadiene copolymer; and
(J) a blend of polyamide 6 and polyamide 6I6T.
This patent application, among hundreds of theoretically possible blends encompassed within the above very broad definition, is focused on blends of ethylene-alfa olefin copolymers and ethylene vinyl acetate copolymer (VLDPE or LLDPE and EVA). These blends are profusely described, exemplified and also specifically claimed (claim 13) while other blends, such as those of ethylene homopolymer and/or ethylene/alpha-olefin copolymer with ethylene/unsaturated ester copolymer, are completely ignored.
In addition to the above mentioned properties heat-shrinkable barrier packaging films for bags should have good optics, hot wire trimming and knife cutting—i.e. the capability to be cleanly cut on the bag making machine without being deformed or torn—and machinability. In particular, a film is said to have good “machinability” if it can be used with a packaging machine without undue creasing, folding, seal pleats, edge curls, or jamming. All these defects of machinability are even more evident with films of lower thickness and/or in case of diagonal seals such as those present in bags of triangular or V shape, commonly used for packaging pieces of hard cheeses such as grana and parmesan.
The Applicant has found that films comprising blends according to the teaching of US2009116768, i.e. blends of ethylene-alfa olefin copolymers and ethylene vinyl acetate copolymer, are not easily machinable, especially on converting machine for V-shaped article: recurrent defects during converting are pluckering—namely curled seals in particular along angled seals—a difficult removal of the waste material interposed between two subsequent triangular bags and, after cut, an improper formation of the bag chain (i.e. curled bags do not align orderly and are not evenly spaced).
An attribute of a film that may contribute to good machinability is a higher level of stiffness of the film. However, an increase in the modulus (i.e., stiffness) of a film may result in undesirably reduced free shrink and appearance characteristics (e.g., gloss, haze, and total transmission) of the film.
We have surprisingly found that by inserting at least one layer of ethylene/unsaturated ester copolymers or their blends with ethylene homopolymer and/or ethylene/alpha-olefin copolymer in conventional barrier shrinkable packaging films unexpectedly balanced properties result In particular these films are characterized by good optics, high-shrink, smooth linear tearing, high abuse resistance and good mechanics, easy trimming and machinability, especially advantageous in the manufacture of triangular or V-shaped articles.